why aren’t my voicemails showing up on my iphone app

You can verify your notification settings by going to Settings > Notifications and looking for phone notifications. Be sure that allow notifications is enabled and adjust the settings to your liking. The information at https://support.apple.com/guide/iphone/change-notification-settings-iph7c3d96bab/ios should be of help.

If you are experiencing difficulty with voicemail and the initial response does not help, contact your cellular carrier, they are responsible for voicemail and its features.